Tl
in your element
Poisonous polonium
Eric Ansoborlo considers the disproportionate potency of polonium compared with its relative
scarcity on Earth.
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Bi
454
Po
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THINKSTOCK/TORSTEN LORENZ
P
olonium is the 84th entry in the periodic
table and is a natural radioelement
present in the environment at extremely
low concentrations. The discovery of
polonium was published1 by Pierre and
Marie Curie in July 1898. It was the first
element discovered using their newly
developed radiochemical separation
method, where each product isolated from
primary pitchblende ore was detected using
an electrometer constructed by Pierre.
They cautiously wrote “we believe that the
substance we recovered from pitchblende
contains a heretofore unknown metal, similar
to bismuth in its analytical properties. If the
existence of this new metal is confirmed,
we propose that it be named polonium in
honour of the native land of one of us.”
This discovery led to the first controversy 2
in radiochemistry, in this case between the
Curies and a number of German scientists
— including Willy Marckwald and Friedrich
Giesel — who thought that this new element
was nothing other than ‘inducedly activated
bismuth’. In 1901 Marckwald isolated, using
a different method, what turned out to be
the same ‘new substance’ and provisionally
renamed it radiotellurium. For several years,
controversy over both the identity of the
material and its location in the periodic
table continued, and it was only in 1910 that
Marie Curie and André-Louis Debierne
unequivocally identified, by a spark spectrum
technique, 0.1 mg polonium in 2 mg of
separated sulfide residue. In 1911 Marie Curie
received the Nobel Prize in Chemistry for the
discoveries of both polonium and radium.
Polonium has 41 isotopes, with masses
ranging from 187 to 227. The predominant
naturally occurring isotope is 210Po, which
is a radioactive decay product in the natural
uranium decay series. Nowadays the two
main methods for producing significant
quantities (that is, a few milligrams) of 210Po
are to neutron-irradiate a 209Bi target in a
nuclear reactor, or to bombard a 209Bi target
with a 37 MeV beam of alpha particles.
Isotope 210Po is almost exclusively an
alpha emitter with a half-life of 138.4 days.
Consequently, it is both highly radioactive
and highly toxic3. Because of its radiotoxicity,
the fact that it volatilizes at low temperature
(~50 °C) and that it sticks rather avidly to
glass, it is difficult to handle, and therefore its
chemistry remains only poorly known4.
Belonging to the chalcogenide family of
the periodic table, along with oxygen, sulfur,
selenium and tellurium, polonium can also
be compared to bismuth, its neighbour to the
left in the periodic table. Of several oxidation
states (-ii, +ii, +iv and +vi), the tetravalent
Po(iv) is the most stable in aqueous solution,
and this gives rise to its most important
chemical property: its tendency to hydrolyse
and form colloids of Po(OH)4, in analogy
with most tetravalent elements. It also forms
soluble salts with chlorides, acetates, nitrates
and other inorganic anions, but precipitates
with sulfides, and this is the property that
was originally exploited by the Curies in
separating and identifying it. With organic
ligands, however, the coordination chemistry
of Po(iv) is much more complicated and leads
to mixed oxide and hydroxide complexes.
Fr
Ra
Ac
Th
Pa
U
Np
Elemental 210Po has a few specific uses,
such as studying the effects of alpha particles
and for the calibration of radiation detectors.
It is also a concentrated source of heat, and
small neutron sources can be prepared by
mixing it with beryllium. It is perhaps most
widely known, though, as a rare but highly
toxic poison that targets the liver 3 and also
affects the bone marrow, gastrointestinal and
central nervous systems. The lethal dose for
humans is estimated to be fewer than 10 μg,
and its radiotoxicity was demonstrated soon
after its discovery, when the accidental release
of polonium from a distillation vessel in the
Curie lab caused the death of a technician.
More recently, Alexander Litvinenko,
a former member of the Russian security
services, was thought to have been poisoned5
with 210Po-contaminated tea in 2006, and the
involvement of polonium has been suggested
in Yasser Arafat’s death. Polonium is over
10,000 times more toxic than hydrogen
cyanide and, alongside the botulinum toxin,
it is one of the most toxic substances known.
In soils, 210Po is absorbed by clay minerals
and organic matter. In particular, it is known
to accumulate in tobacco plants, which
results in a surprisingly significant presence
of polonium in cigarettes. Not good news for
smokers then, considering that only a few
studies have been carried out into treatments
for polonium poisoning, with thiol-based
chelating agents the only recommended
ones thus far.
❐
ERIC ANSOBORLO is in the Department
of Radiochemistry and Processes, French
Alternative Energies and Atomic Energy
Commission, Marcoule Center, 30207
Bagnols-sur-Ceze, France.
e-mail: [email protected]
References
1. Curie, P. & Skłodowska-Curie, M. C. R. Acad Sci.
127, 175–178 (1898).
2. Adloff, J.‑P. & Kauffman, B. Chem. Educ. 12, 94–101 (2007).
3. Ansoborlo, E. et al. Chem. Res. Toxicol. 25, 1551–1564 (2012).
4. Bagnall, K. W. et al. (eds) Gmelin Handbook of Inorganic and
Organometallic Chemistry: Po-Polonium (Springer, 1990).
5. Jefferson, R. D., Goans, R. E., Blain, P. G. & Thomas, H. S. Clin.
Toxicol. 47, 379–392 (2009).
Pu Am Cm Bk
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NATURE CHEMISTRY | VOL 6 | MAY 2014 | www.nature.com/naturechemistry
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